It is known that the production of calcium is done especially by aluminothermic treatment of lime at about 1400.degree. C. in the solid phase, by the following reaction: EQU 5CaO+2Al.fwdarw.Al.sub.2 O.sub.3, 2CaO+3Ca
The reaction is carried out in an oven in a vacuum. The calcium is separated in the vapor state and condensed to a solid.
However, aluminum vapor is entrained in such a way that this process leads to a calcium that contains a not insignificant quantity of aluminum, on the order of from one to several percent, in the form of CaAl.sub.2.
The production of calcium by electrolysis of a calcium salt is also known. This process produces a calcium with a low content of impurities, in particular aluminum, but it is not economically profitable compared with the previous process.
Finally, it has also been proposed that calcium be produced by the following reaction: EQU CaSi+Fe.fwdarw.FeSi+Ca
This reaction, which may be done in the solid phase with divided products, or in the liquid phase after melting of the starting products, does produce a calcium of adequate purity, but is no longer competitive economically compared with the first method discussed above, that is, aluminothermic treatment of lime.